1. Field of the Invention
This invention relates to gravity filters having systems for distributing backwash fluids throughout a bed of granular media and, more particularly, to means for regulating the velocity of backwash liquids to ensure their uniform distribution into the filter media.
2. Description of the Prior Art
Gravity filters are used for various purposes, including filtering wastewater for a municipal water supply. The filters typically include a bed of granular media which is supported by an underdrain, and a flume receives filtered effluent from the underdrain and carries it away from the filter. Periodically, the granular media must be cleansed by initiating a "backwash" operation. During backwash, flow through the filter is reversed, and backwash liquid, typically filtered water, enters the flume which then distributes it to the underdrain. The underdrain distributes the backwash water into the granular media.
The underdrain usually includes a plurality of underdrain laterals which transverse the bottom of the filter and which may be formed from folded metal sheets, molded plastic blocks, or blocks and/or plates made from ceramic materials. Typical gravity filter underdrains are discussed in U.S. Pat. Nos. 2,154,167; 3,110,667; 3,468,422; 3,956,134; 4,065,391; 4,214,992 and 4,331,542. The flume may be recessed in the center of the filter floor, or it may be located behind one of the filter end walls.
To ensure proper performance of the gravity filter, it is critical that the backwash water is uniformly distributed to the bed of granular media. The degree to which the backwash system uniformly distributes water is conversely referred to as maldistribution. In newly constructed gravity filters, flume dimensions can be specified to keep backwash water velocities under two feet per second. At these velocities, maldistributions of .+-.5% over the top surface of the underdrain are readily obtained.
However, older filters have fixed flume dimensions which necessitate backwash water velocities as high as ten feet per second. As this high-speed flow proceeds down the flume and enters each lateral, the velocity becomes progressively lower. Since typical lateral inlets are all made with the same cross-sectional area, this variation in velocity and pressure head causes significant maldistribution into the laterals, easily in the range of .+-.20-30%. Laterals toward the far end of the flume tend to fill up before those near the flume inlet. This maldistribution is carried over into the filter bed, causing media shifts, inadequate cleansing and other problems. Although the laterals themselves are usually designed to provide minimal maldistribution, the high maldistribution lateral inlet negates these advantages.
Hudson, Jr., "Water Clarification Processes Practical Design and Evaluation", Chapter 15 (1981) discloses a theory for varying the lateral inlet areas to reduce maldistribution. Hudson discusses "dividing-flow manifolds" which are analogous to flow patterns through a flume and into underdrain laterals. Essentially, Hudson suggests successive reduction of lateral inlet area along the manifold, but he notes that this can cause construction difficulties. He also suggests "adjustable gates" but states that plant operators find these unsatisfactory. For new construction jobs, Hudson believes that tapering the manifold to decrease the cross-sectional area, and hence keep a constant velocity, is the best solution. However, this solution limits future expansion alternatives and is clearly not suitable to retrofit jobs with fixed flume dimensions.
As Hudson theorized, different sized orifices may be placed in front of each lateral to equalize flow into the laterals, with larger orifices close to the flume inlet and smaller orifices toward the back of the flume. While the orifice sizing is calculable, many practical variables cannot be accounted for in the theoretical calculation. Due to the peculiarities of each gravity filter, an ideal system would permit convenient trial and error sizing of lateral inlets to ensure that backwash water enters each lateral at the same velocity, thus obtaining optimum backwash distribution from the underdrain into the bed of granular media. At present, this can only be done manually by entering the flume after the underdrain laterals have been installed to affix new orifices at each lateral inlet. Current underdrain lateral designs, which may require the application of adhesives and grouts, are not conducive to successive installment, removal and reinstallment of underdrain lateral components, and manual work within the confined flume may be hazardous. Certain flumes are even too small for a man to enter. Moreover, with the current state of the art, even if proper stationary orifices could be conveniently installed to equalize flow in the underdrain lateral during backwash, an uneven flow distribution during the filtration cycle would be created when flow is in the opposite direction.
Therefore, it is an object of the present invention to provide a flume distribution system which allows trial and error lateral inlet sizing without the necessity of entering the flume or replacing the underdrain laterals themselves. Additionally, it is an object of the invention to provide such a system without disturbing flow patterns when the filter is in the filtration mode. It is a still further object to provide such a system which is adaptable to a wide variety of flume configurations, for example, central flume, end flume and other designs.